One manner of drilling curved sections of well holes is to place a downhole motor assembly at an end of a drill string. The downhole motor assembly imparts a rotational force to a drill bit. The downhole motor assembly includes a "bent" housing portion that is offset at an angle. When the drill string is stationary the downhole motor assembly drills at an angle dictated by the offset in the housing along a radiused curve. Techniques have been developed for "steering" the downhole motor assembly. It has been determined that by rotating the drill string the downhole motor assembly can be made to drill straight ahead. It must be appreciated that horizontal wells have to be accurately placed in three dimensions in order to reach an intended target zone. Sophisticated measuring while drilling systems (MWD) have been developed that provide timely survey and drill bit orientation readings, so that corrections can be made while drilling. This ability to "steer" to the intended target zone saves an enormous amount of time and money.
There is a limit on the radius of curve that can be drilled from vertical to horizontal using the described "steering" technique. There has been a push within the industry to create shorter radiused drilling. The definition of what constitutes a "short radius" is constantly been redefined. U.S. Pat. No. 4,442,908 which issued to the Preussag Aktiengeselischaft firm of Germany in 1984 describes what is entitled a "Tool for Drilling Curved Sections of Well Holes". In the description of prior contained in the Preussag patent the following comment is made regarding the radii of curvature available at that time;
"the radii of curvature are relatively large, so that deviations of, say, 90 degrees are attainable only by traversing vertical depths on the order of 600 to 1000 meters."
The Preussag patent proposes the use of a segment of drill string flexible in one plane only. This flexible segment of drill string consists of a series of generally-tubular individual link members pivotally connected to each other by means of pins.
In 1992 Eastman Christensen issued a brochure describing their long radius, medium radius and short radius, horizontal drilling systems. This brochure maintained that their short radius system "can turn a well from vertical to horizontal in 30-60 feet along radii of 20-40 feet". The approach taken by Eastman Christensen appears to be similar to that proposed in the Preussag patent. A "flexible drive pipe" is described and illustrated. This flexible drive pipe appears to consist of a series of generally-tubular individual link members pivotally connected to each other.
The problem with the solutions proposed in the Preussag Patent and subsequently by Eastman Christensen Company is that both publications indicate that when their flexible drive systems are employed the drill string must not be rotated once the drilling motor assembly is oriented horizontally. It will be appreciated that the inability to rotate the drill string severely limits the ability to "steer" the downhole motor assembly to the target zone.